In the induction field, it has been known that when an inductor carrying alternating current is positioned in proximity to a conducting surface, a current will flow within that conducting surface producing heating effect proportional to the square of the current density within the conducting surface. If conductors carrying currents of the same phase are placed opposite each other on each side of a strip of conducting metal, it would produce a magnetic field that is essentially perpendicular to the strip. This type of heating that results, has been described as transverse flux heating.
The term "transverse flux heating" as used herein applies to all configurations wherein the axis of the induction coil is substantially perpendicular to the plane of the sheet so that the resulting electromagnetic flux will also tend to be in a direction transverse to said plane. By contrast in conventional induction heating, a solenoidal coil surrounds the sheet with its axis parallel to said plane, and the resulting flux is parallel to said plane, and also parallel to the direction of the continuous motion of the sheet.
Another distinguishing mark of transverse flux heating devices is the use of magnetic flux guides in conjunction with the conductors of the induction coil. These flux guides are disposed so as to further enhance the flux in a direction transverse to said sheet.
The entire flux produced by a transverse flux heating coil will pass through the sheet from one side to the other only if the frequency of the alternating current applied to the coil is relatively low. As said frequency is increased, currents induced in the sheet produce an increasingly significant counter-flux, so that only a fraction of the primary coil flux transverses the sheet. If the frequency is further increased to extremely high values, entirely separate flux patterns will form on each side of the sheet, and virtually no flux passes through the sheet. External to the sheet, the direction of the flux then will still be transverse but within the strip most of the flux lines will be parallel to the plane of the sheet. This phenomenon, not withstanding, the term transverse flux heating is applied to all devices which produce a substantially transverse flux when no sheet is present.
It can also be shown that at relatively low frequency, the current density at any one point on the sheet remains substantially constant throughout the thickness of the sheet even if an inductor is applied only to one side of the strip. At relatively high frequency, however, the current density at any one point of the sheet will greatly decrease from the surfaces inward, and then it will be necessary to apply inductors to both sides of the strip to obtain uniform temperature in the strip.
The actual range of frequencies producing these changing effects in a given sheet will depend on the thickness, electrical resistivity, magnetic permeability, configuration of the heating coil, and density of the induced current. Most practical transverse flux heating applications will be found to lie between the extremes cited above, i.e., the operating frequency is such that a significant fraction of the flux will transverse the sheet, but the current distribution over the thickness at any one point will be non-uniform and therefore an installation utilizing a coil on one side of the strip may produce non-uniform temperature distribution through the thickness of the strip.
Robert E. Baker, in U.S. Pat. No. 2,448,012, attempts to utilize transverse flux heating to process continuously moving strip. Because of difficulty in heating the strip uniformly, Baker attempts to modify the field distribution by introducing shielding means at the strip edges. The use of shielding means causes loss of energy and decreases the efficiency of the transverse flux heater. Lackner, U.S. Pat. No. 2,902,572, also attempts to obtain uniform heating and disposes a core structure arranged so that strip passing therethrough is heated by a current which passes diagonally across the width but does not secure uniform heating, particularly for variable width strip.
Other inventors such as Jackson, et al, U.S. Pat. No. 4,054,770, have addressed the said problem by suggesting the use of wedge shaped appendages on the surface of the pole faces used in the transverse inductor assembly. In the same reference, it is suggested to use arrays formed from multiple inductors of the same construction to produce a uniform heating effect. Neither construction of Jackson's lends itself to uniform heating of strips of variable widths.
Throughout the world, few commercial installations utilizing transverse flux heating of strip have been made and those have been limited to fixed width strip and utilize relatively low power density.